166 BA CTEBIOL OGY. 



clear. To 100 c.c. of the clear filtrate add 10 c.c. of 

 absolute alcohol and 11 c.c. of the concentrated alco- 

 holic solution of either fuchsin, methylene-blue, or gen- 

 tian-violet, preferably fuchsin or gentian-violet. 

 Ziehl's earbol-fuchsin solution : 



Distilled water . . . 100 c.c. 



Carbolic acid (crystallized) ... .5 grammes. 



Alcohol . ... 10 c.c. 



Fuchsin in substance . . .... 1 gramme. 



Or it may be prepared by adding to a 5 per cent, 

 watery solution of carbolic acid the saturated alcoholic 

 solution of fuchsin until a metallic lustre appears on 

 the surface of the fluid. 



The Koch-Ehrlich solution decomposes after a time, 

 so that it is better to prepare it fresh in small quantities 

 when needed than to employ old solutions. Solutions 

 older than fourteen days should not be used. 



The three solutions just given may be used for cover- 

 glass preparations in the ordinary way. 



In some manipulations it becomes necessary to stain 

 the bacteria very intensely, so that they may retain 

 their color when exposed to the action of decolorizing 

 agents. These methods are usually employed when it 

 is desirable to deprive surrounding objects or tissues of 

 their color, in order that the stained bacteria may stand 

 out in greater contrast. It is in these cases that the 

 staining-solution with which the bacteria are being 

 treated is to be warmed, and in some cases boiled, so as 

 further to increase its penetrating action. When so 

 treated, certain of the bacteria ^\\ll retain their color, 

 even when exposed to very strong decolorizers. The 

 tubercle bacillus is distinguished from the great ma- 



